# Modelling ultraviolet-line diagnostics of stars, the ionized and the neutral interstellar medium in star-forming galaxies

## TL;DR

This paper develops a comprehensive model to analyze ultraviolet-line diagnostics in star-forming galaxies, accounting for stellar radiation, nebular emission, and interstellar absorption, to better interpret galaxy spectra.

## Contribution

It introduces a physically consistent approach to model the combined effects of stars, ionized, and neutral ISM on ultraviolet spectra of star-forming galaxies, improving diagnostic accuracy.

## Key findings

- Most UV indices are contaminated by ISM effects, especially at higher metallicity.
- Certain nebular and interstellar features serve as cleaner tracers of ISM phases.
- Neglecting stochastic sampling in young clusters has minimal impact on age and metallicity estimates.

## Abstract

We combine state-of-the-art models for the production of stellar radiation and its transfer through the interstellar medium (ISM) to investigate ultraviolet-line diagnostics of stars, the ionized and the neutral ISM in star-forming galaxies. We start by assessing the reliability of our stellar population synthesis modelling by fitting absorption-line indices in the ISM-free ultraviolet spectra of 10 Large-Magellanic-Cloud clusters. In doing so, we find that neglecting stochastic sampling of the stellar initial mass function in these young ($\sim10$-100 Myr), low-mass clusters affects negligibly ultraviolet-based age and metallicity estimates but can lead to significant overestimates of stellar mass. Then, we proceed and develop a simple approach, based on an idealized description of the main features of the ISM, to compute in a physically consistent way the combined influence of nebular emission and interstellar absorption on ultraviolet spectra of star-forming galaxies. Our model accounts for the transfer of radiation through the ionized interiors and outer neutral envelopes of short-lived stellar birth clouds, as well as for radiative transfer through a diffuse intercloud medium. We use this approach to explore the entangled signatures of stars, the ionized and the neutral ISM in ultraviolet spectra of star-forming galaxies. We find that, aside from a few notable exceptions, most standard ultraviolet indices defined in the spectra of ISM-free stellar populations are prone to significant contamination by the ISM, which increases with metallicity. We also identify several nebular-emission and interstellar-absorption features, which stand out as particularly clean tracers of the different phases of the ISM.